Devices for locating avalanche victims operate with an unmodulated transmission signal at 457 kHz. The normal procedure for skiers is that all the members of a group switch their devices to transmitter operation. Then if part of the group is caught in an avalanche, the others switch their devices to receive mode and try to locate the buried ones on the basis of the signals their devices are transmitting.
The transmission signal is pulsed at a frequency of about one hertz. The transmission time at the frequency of 457 kHz, the so-called duty cycle, is from ten to thirty percent.
For localization by hearing (e.g., maximal/minimal field strength) conventional devices generate an audible search tone at a frequency of about 2 kHz, by down-mixing of the 457-Hz transmission signal. Because the built-in antenna has a pronounced directional characteristic, by rotating the receiving device and looking for the loudness maximum or minimum it is possible to detect the direction at which the strength of the signal emitted by the buried transmitter is maximal. This technique demands experience and close concentration by the searcher, as well as a low level of ambient noise, especially where long distances are concerned.
To simplify the search even for searchers who have had no previous experience and are in stress situations, devices have been developed with several antennae disposed at right angles to one another. By switching between these antennae, the direction from which the transmitted signal is being received can be determined.
In practice, this method has a number of disadvantages. For one thing, the antennae influence one another even when turned off, so that the reception sensitivity of the device deteriorates. In particular, it is almost impossible to determine directionality in the case of large distances, over 50 meters, so that the directional indications thus obtained are not usable. Another disadvantage is that this technique is extremely sensitive to disturbances, so that the indicated direction varies widely when conditions are not optimal.
A particular challenge is presented to the searcher when the signals sent by several buried transmitters are being received simultaneously. In this case an extraordinary amount of practice as well as a complicated search strategy are needed in order to localize the sender.